This application relates to cell search and signal synchronization in wireless communication systems including systems based on orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA).
Wireless communication systems use a network of base stations to communicate with wireless devices registered for services in the systems. Each base station emits radio signal that carry data such as voice data and other data content to wireless devices. Such a signal from a base station can include overhead load other than data for various communication management functions, including information to allow a wireless device to identify a cell sector of a base station, to synchronize signaling in time and frequency. Each wireless device processes such information in the overhead load of reach received signal prior to processing of the data.
OFDM and OFDMA based communication systems are based on the orthogonality of frequencies of multiple subcarriers and can be implemented to achieve a number of technical advantages for wideband wireless communications, such as resistance to multipath fading and interference. However, many OFDM and OFDMA based wireless communication systems can be sensitive to frequency offsets and phase noise. In an OFDM or OFDMA based wireless communication system, the wireless service to a geographic area is provided by dividing the area into a plurality of cells, which can be further divided into two or more cell sectors. The base stations, which conceptually locate at the center of respective cells of their coverage, transmit information to a mobile subscriber station (MSS) via downlink (DL) radio signals sent out from the base stations. A mobile subscriber station is also known as a mobile station (MS) or the wireless station. The mobile stations transmit information to their serving base stations via uplink (UL) radio signals.
The downlink radio signals from the base stations to mobile stations may include voice or data traffic signals or both. In addition, the base stations generally need to transmit preamble signals in their downlink radio signals to identify to the mobile stations, the corresponding cells and corresponding segments in the cells from which the downlink radio signals are transmitted. Such a preamble signal from a base station allows a mobile station to synchronize its receiver in both time and frequency with the observed downlink signal and to acquire the identity, such as IDcell and Segment, of the base station that transmits the downlink signal.
IEEE 802.16 OFDMA has been developed to provide wireless communications based on an orthogonal frequency division multiple access (OFDMA) modulation technique. In the DL preambles currently defined in IEEE 802.16 OFDMA, the MSSs store predefined and handcrafted pseudo-noise (PN) like sequences for identifying IDcell numbers and segment numbers of the adjacent cells. In operation, an MSS captures the preamble symbols in received downlink signals and correlates the preamble in each received downlink signal with the stored pseudo-noise (PN) like sequences to determine IDcell and Segment of a specific sector for that received downlink signal. These preamble sequences are handcrafted in advance and are processed by the MSS one at a time. There are more than 100 such sequences (e.g., 114) in some implementations of the current IEEE 802.16 OFDMA. Performing the cross-correlation with such a large number of preamble sequences can be time consuming, and thus can adversely affect the quality of service to a mobile station, especially when the mobile station is rapidly moving.
FIG. 1 shows an example of the signal subframe format in the time domain for an OFDMA downlink signal in an OFDMA system. This subframe structure is defined in IEEE P802.16-REVd/D5-2004 standard and includes a number of sequential OFDM symbols 110, 120, 130, and 140. Each OFDM symbol has a cyclic prefix (CP) (112 or 142) and a fast Fourier transform (FFT) symbol (114 or 144) which is the inverse FFT (IFFT) of the payload sequence in frequency. The CP 112, 142 is a copy of the last portion 116, 146 of the FFT symbol 114, 144 that the CP 112, 142 is prefixed to. The CP 112, 114 is used to combat adverse multipath effects in a DL signal received at a mobile station.
In the illustrated example, the first OFDM symbol 110 in the downlink subframe contains a payload of the preamble in frequency. Each of the subsequent OFDM symbols 120, 130, and 140 contains a payload of data in frequency. The BS transmits the cell-specific preamble symbol 110 in each downlink subframe in order for the MSS receiver to synchronize with the received downlink signals in both time and frequency, and to perform cell search, cell selection, and cell reselection.
FIG. 2 illustrates an exemplary detection procedure in an MSS under IEEE P802.16-REVd/D5-2004. In this preamble detection scheme, the MSS receiver first performs time synchronization in step 202 by performing a CP correlation procedure. Once the time is synchronized, the CP 112 is removed and the FFT operation is performed in step 204 on the remaining time sequence that has a length of the FFT size in order to restore the payload sequence in frequency. In step 206, the output sequence of the FFT operation is correlated with each of all preset cell-specific preamble sequences in frequency, one sequence at a time. For example, in a system with 114 cell-specific preamble sequences, the correlation procedure is performed 114 times. Next in step 208, the MSS receiver determines whether the largest correlation output from step 206 is greater than a predetermined detection threshold. If the largest correlation output is greater than the threshold, the cell-specific preamble sequence corresponding to the largest correction output is identified and the associated BS is selected as the current serving BS. Next, the MSS receiver further processes the rest of the downlink subframe signal such as extracting the data in the data symbols. If the largest correlation output is not greater than the detection threshold, the MSS receiver moves on to the next received OFDM symbol and repeat the steps from step 202 to step 208 to search for a correlation output greater than the correlation threshold.
IEEE P802.16-REVd/D5-2004 has 114 unique preamble sequences to represent up to 114 combinations of BS cell sites and antenna segments. Table 1 below shows a portion of the 114 preamble sequences in frequency, with the associated cell identities (i.e. IDcell and Segment). Each preamble sequence is a handcrafted pseudo-noise (PN) sequence that has relatively good autocorrelation characteristics in the frequency domain and low peak to average power ratio (PAPR) in the time domain. However, it is time- and power-consuming to perform the correlation process for all 114 candidate preamble sequences. Also, because the CP is only a fraction of the FFT size, the CP based symbol timing detection method may not be sufficiently accurate and may introduce inter-symbol interference (ISI), thus degrading the performance of the cell search process that follows. Furthermore, the initial frequency estimation based on CP correlation can be coarse and thus may not be reliable. These technical limitations of the preamble design under IEEE P802.16-REVd/D5-2004 can lead to a long cell search time, which may not be acceptable in some communication applications, such as wireless mobile communication services.
TABLE 1PreambleIndexIDcellSegmentPreamble Sequences (in Hexadecimal format)000A6F294537B285E1844677D133E4D53CCB1F182DE00489E53E6B6E77065C7EE7D0ADBEAF110668321CBBE7F462E6C2A07E8BBDA2C7F7946D5F69E35AC8ACF7D64AB4A33C467001F3B22201C75D30B2DF72CEC9117A0BD8EAF8E0502461FC07456AC906ADE03E9B5AB5E1D3F98C6E............313104841AFC277B86A0E067AF319422F501C87ACBFBDD66BFEA3644F879AE98BA8C5D6051233201F35EA87318E459138A2CE69169AD5FD9F30B62DA04ED21320A9F59893F0D176752152FD3311A0C5F35C5971CD3DC55D7D2B9FD27AA17A198583F580EB0800744EE5B6B3648DEA958403421A6D3D33AD9B56862DBF076E3ACE6A3150510CCC8BE77DE4E6E10EB5FE163765647D07DF............95312E7FDDCEED8D31B2C0752D976DE92BEA241A713CF818C274AA1C2E3862C7EB7023AF35D496000D26B8D5452948E30FD29D36E8404C8456657A6CBEEFCD91BB14F91E291F2C1C8F4119F97112CC0EEF167D75102669A814D11A51E569D07A7433A9762A292D7E2A4FE35B9130FD67DE9822FB5CBD0CB6FA80C8B3560B4C443BA4900BF2729B160098C2F783A7752B8BA235010A1DA993087BF4954022D30549DF7348477EACB97AC3565B838460CC62F242883313B15C3137033510041076BF72542FB9FEBDEC3C316BC28CF0607BCEC5399EC9228905375D3D15F929B586D7E6............110142503F196BBF93C238BFD5E735E5AE52E0DAE64F5E2F4C3B92E553F51303C4A64C4403BF3111150CA346FCCF511822F524C043D2003F3B12DCFEA276B91BA98EB3984BBCFDF75C2A2E1B271121615FD4A6894566678C95B9D5A59DDE5366799045FEB03A2BAA74094140E9068C61C2E972C113172E68AE62D2EE51B14F9D33ED737253CDB55A6E277254050830F2FE409E5EE284534FC3E5